As a detector for a gas chromatograph, various types of detectors have been practically applied, such as a thermal conductivity detector (TCD), electron capture detector (ECD), flame ionization detector (FID), flame photometric detector (FPD), and flame thermionic detector (FTD). Among these detectors, the FID is most widely used, particularly for the purpose of detecting organic substances. The FID is a device that ionizes sample components in a sample gas by hydrogen flame and detects the resultant ion current. It can attain a wide dynamic range of approximately six orders of magnitude. However, the FID has the following drawbacks: (1) Its ionization efficiency is low, so that its minimum detectable amount is not sufficiently low; (2) Its ionization efficiencies for alcohols, aromatic substances, and chlorine substances are low; (3) It requires hydrogen, which is a highly hazardous substance; therefore, an explosion-proof apparatus or similar kind of special equipment must be provided, which makes the entire system difficult to operate.
On the other hand, a pulsed discharge detector (PDD) has conventionally been known as a detector capable of high-sensitivity detection of various compounds from inorganic substances to low-boiling-point organic compounds, (for example, see Patent Document 1). In the PDD, the molecules of helium or another substance are excited by a high-voltage pulsed discharge. When those molecules return from their excited state to the ground state, they generate light energy. This light energy is utilized to ionize a molecule to be analyzed, and an ion current produced by the generated ions is detected to obtain a detection signal corresponding to the amount (concentration) of the molecule to be analyzed.
In most cases, the PDD can attain higher ionization efficiencies than the FID. For example, the ionization efficiency of the FID for propane is no higher than 0.0005%, whereas the PDD can achieve a high level of approximately 0.07%. However, the dynamic range of the PDD is not as wide as that of the FID; the fact is that the former is lower than the latter by one or more orders of magnitude. This is one of the reasons why the PDD is not as widely used as the FID.
The most probable constraining factors for the dynamic range of the conventional PDD are the unstableness of the plasma created for the ionization and the periodic fluctuation of the plasma state. To solve this problem, a discharge ionization current detector has been proposed (for example, refer to Patent Document 2). This detector uses a low-frequency AC-excited dielectric barrier discharge (which is hereinafter referred to as the “low-frequency barrier discharge”) to create a stable and steady state of plasma. The plasma generated by the low-frequency barrier discharge is non-equilibrium atmospheric pressure plasma, which does not become hot as easily as the plasma created by the radio-frequency discharge. Furthermore, the periodic fluctuation of the plasma, which occurs due to the transition of the voltage application state if the plasma is created by the pulsed high-voltage excitation, is prevented, so that a stable and steady state of plasma can be easily obtained. Based on these findings, the present inventors have conducted various kinds of research on the discharge ionization current detector using a low-frequency barrier discharge and have made many proposals regarding this technique (for example, refer to Patent Documents 3 and 4).
FIG. 5 shows one example of the conventional configuration of the discharge ionization current detector using a low-frequency barrier discharge.
This discharge ionization current detector includes a cylindrical tube 41 made of a dielectric material, such as quartz, with its inner space serving as a first gas passage 42. Ring-shaped plasma generation electrodes 43, 44 and 45 made of a metal (e.g. stainless steel or copper) are circumferentially provided at predetermined intervals on the outer wall surface of the cylindrical tube 41. According to this design, the dielectric wall of the cylindrical tube 41 between the first gas passage 42 and the plasma generation electrodes 43, 44 and 45 serves as a dielectric coating layer that covers the electrodes 43, 44 and 45, and thereby enables dielectric barrier discharge to occur.
Among the three plasma generation electrodes 43, 44 and 45, the central electrode 44 is connected to an excitation high-voltage power source 55 for generating a low-frequency high AC voltage, while the other electrodes 43 and 45 located on both sides of the central electrode 44 are connected to the ground.
In the lower portion of the cylindrical tube 41, a recoil electrode 50, a bias electrode 52 and an ion-collecting electrode 53 are arranged, with insulating members 51 provided between them. Each of these electrodes consists of a cylindrical body having the same inner diameter. These cylindrical bodies internally form a second gas passage 49 continuously extending from the first gas passage 42 in the cylindrical tube 41. Therefore, these electrodes 50, 52 and 53 are directly exposed to the gas inside the second gas passage 49. The recoil electrode 50, which is connected to the ground, prevents the charged particles in the plasma from reaching the ion-collecting electrode 53, whereby the noise is reduced and the S/N ratio is improved. The bias electrode 52 is connected to a bias DC power source 61, while the ion-collecting electrode 53 is connected to a current amplifier 63 included in an ion current detector 62. In the second gas passage 49, the space inside the bias electrode 52, the ion-collecting electrode 53 and the intervening section corresponds to the substantial current-collecting area.
A gas supply tube 46 is connected to the upper end of the cylindrical tube 41. Through this gas supply tube 46, a predetermined gas is supplied from a gas supply source (not shown) into the first gas passage 42. A first exhaust tube 47 is connected to the recoil electrode 50, which is located at the connecting portion between the first and second gas passages 42 and 49, while a second exhaust tube 48 is connected to the dead end of the second gas passage 49. A thin sample introduction tube 54 is inserted in the second gas passage 49. Through this sample introduction tube 54, a sample gas containing a sample component to be analyzed is supplied into the second gas passage 49.
A detecting operation by this discharge ionization current detector is hereinafter described. As shown by the right-pointing arrow in FIG. 5, a plasma gas, which doubles as a dilution gas, is supplied through the gas supply tube 46 into the first gas passage 42. The plasma gas doubling as the dilution gas flows downward through the first gas passage 42. At the lower end of the first gas passage 42, a portion of this gas is separated, to be eventually discharged through the first exhaust tube 47 to the outside. The remaining portion of the plasma gas serves as the dilution gas to be mixed with the sample gas and flows into the current-collecting area.
While the plasma gas is flowing through the first gas passage 42 in the previously described manner, the excitation high-voltage power source 55 is energized, whereupon the excitation high-voltage power source 55 applies a low-frequency high AC voltage between the plasma generation electrode 43 and each of the other electrodes 44 and 45. As a result, electric discharge occurs in the plasma generation area between the electrodes 44 and 45 in the first gas passage 42. This discharge is a dielectric barrier discharge since it occurs through the dielectric coating layer (i.e. the cylindrical tube 41). Due to this dielectric barrier discharge, the plasma gas flowing through the first gas passage 42 is ionized over a wide range, producing a cloud of plasma (i.e. atmospheric non-equilibrium micro-plasma).
The atmospheric non-equilibrium micro-plasma emits excitation light, which passes through the first gas passage 42 and the second gas passage 49 to the area where the sample gas exists, and ionizes the molecules (or atoms) of the sample component in the sample gas. Due to the effect of the bias DC voltage applied to the bias electrode 52, the generated sample ions give electrons to or receive electrons from the ion-collecting electrode 53. As a result, an ion current corresponding to the amount of the generated sample ions, i.e. the amount of the sample component, is sent to the current amplifier 63, which amplifies the current and outputs it as the detection signal. In this manner, the present discharge ionization current detector produces a detection signal corresponding to the amount (concentration) of the sample component contained in the introduced sample gas.